Technical Field
The present disclosure relates to methods of removing iron sulfide scale deposits from surfaces in fluid communication with a wellbore and/or subterranean formation, and more particularly to methods for removing such deposits with a composition comprising at least one chelating agent and at least one converting agent.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly nor impliedly admitted as prior art against the present invention.
During the production stage of a wellbore, fluids (e.g., gas, oil, steam, hot water, etc.) are generally produced from the wellbore, and scale can develop in the wellbore, subterranean formation and/or on equipment associated with the wellbore, such as downhole equipment (e.g., casings, production tubing, mandrels, pipes, pumps, etc.) and surface equipment (e.g., pumps, heating turbines, heat exchangers, etc.). Whenever the wellbore produces water, or when water injection is used to enhance the recovery of the natural resource, there is the possibility that scale will form. One type of scale includes iron sulfide compounds, which have a physical appearance of amorphous solid particles capable of absorbing water and oil.
Hydrogen sulfide, H2S, is a naturally occurring contaminant of fluids that is encountered in many industries, including the oil and gas industry and the paper industry. The corrosive nature of H2S causes the accumulation of particulate iron sulfide. Iron sulfide becomes entrained in hydrocarbons, glycol, salts, and the like to form scale deposits on the surfaces of conduits such as pipelines. Such deposits present a significant problem because the deposits hinder accurate determinations of pipeline structural integrity and the pipelines must be cleaned physically.
Given the various chemical and physical conditions that go into the forming of iron sulfide scales, several forms can be found in a given section of a wellbore and a pipeline. It is seldom that a single type of iron sulfide scale exists; but more generally it is a mixture of iron sulfide scales, including pyrrhotite (Fe7S8), troilite (FeS), marcasite (FeS2), pyrite (FeS2), greigite (Fe2S4), and mackinawite (Fe9S8).
The iron sulfide particles can adhere to the internal surfaces of a wellbore, pipeline networks, and associated process equipment. The physical characteristic of the iron sulfide scale deposits can vary from a viscous, oil coated mass to a dry black powder form. The buildup of iron sulfide scale deposits over time can lead to a range of operational problems at a wellbore's completion and production stages, from inhibiting the performance of downhole tools to inducing formation damage and plugging of associated equipment, whether such equipment is located downhole or at the surface of the wellbore. The iron sulfide scale deposits may block flow of the natural resources by clogging perforations or forming a thick lining in the production tubing. The iron sulfide scale deposits can also coat and damage wellbore equipment, such as heating turbines, heat exchangers, safety valves, casings, production tubing, mandrels, pipes, separators, pumps, etc. If the iron sulfide scale deposits are not removed, the wellbore production capacity can diminish drastically, and in some cases the iron sulfide scale deposits can cause the wellbore production to be shut down for a period of time. Additionally, the iron sulfide scale deposits can lead to increased corrosion rates within pipeline networks and interference in the safe operation of pipeline valving systems, potentially leading to catastrophic system failures. As a result, in the oil industry, the iron sulfide scale deposits are a major source of economic loss. The deposits obstruct the flow of oil in wells, in the adjacent strata and in pipelines as well as in processing and refinery plants. Further, such deposits tend to stabilize oil-water emulsions that tend to form during secondary oil recovery.
Methods have been developed to decrease and remove iron sulfide scale deposits, including batch chemical cleaning, continuous chemical cleaning, and mechanical efforts, such as milling, high pressure water jetting, and sand blasting. The use of a strong acid, such as 10-20 wt % HCl, is the simplest way to dissolve iron sulfide scales, however, it is ineffective in dissolving the pyrite (FeS2) and marcasite (FeS2) forms of the iron sulfide scales. Additionally, using a strong acid generates large volumes of highly toxic H2S gas, which is an undesirable by-product. The strong acid can also have corrosive effects on the wellbore equipment and may damage the formation. Using an oxidizing agent may avoid such toxicity hazards but produces oxidation products, including elemental sulphur which is corrosive to pipes. Another agent for treating such deposits is acrolein, but it also has health, safety and environmental problems. Mechanical methods, such as milling and water jetting using pressures in excess of 140 MPa (with and without abrasives), generally require that each pipe or piece of equipment be treated individually with significant levels of manual intervention, which is both time consuming and expensive, but sometimes also fail to thoroughly remove the iron sulfide scale deposits.
Therefore, there is an urgent need for an iron sulfide removal composition and method which is effective and efficient for all forms of iron sulfide scales, particularly the pyrite and marcasite forms that are insoluble in strong acid, non-corrosive to the equipment, and low cost, and which does not cause damage to the formation and the environment.